The present invention relates to fiber-optic sensors, and particularly to distributed fiber-optic sensor arrays which utilize time division multiplexing in their operation.
Over the past few years, fiber-optic devices have been actively studied and developed for use in various sensing applications in a wide range of fields. One reason for this interest is the sensitivity of optical fibers to environmental conditions which surround them. For example, factors such as temperature, pressure, and acoustical waves directly affect the light transmitting characteristics of optical fiber. These changes in the optical fiber produce a change in the phase of light signals traveling in the fiber. Thus, a measurement of the change in phase of optical signals which have been transmitted through that fiber is representative of changes in those environmental conditions which have affected the fiber.
Recently, particular efforts have been directed to the development of systems having sensors organized in arrays, so that a number of sensors can utilize light from a single source, and provide environmental information at a common detection location. Ideally, such an array would consist of a fiber input bus which would carry light to a set of sensors. Each sensor would imprint information about the environment to this optical carrier. An output fiber bus would then collect this information and bring it back to a central processing location, where information obtained from any selected one of the sensors could be readily identified and analyzed.
The goal of these development efforts is to produce sensor arrays which could be used for specific applications such as monitoring rapidly changing environmental conditions. For example, such sensor arrays could be used to detect acoustic waves in order to determine the source location and acoustical characteristics of those waves. For many such applications, it may be necessary to space the arrays over a relatively large area. In these situations, the replacement of electrical lines by fiber optics, for example, would overcome problems such as electrical pickup, cable weight, and safety hazards associated with the use of those electrical lines. Even when the sensor is used in limited space, the removal of electronics and bulk optics components generally should provide improved system performance due to reduced noise. On the other hand, replacement of long electrical lines by optical fibers creates a problem in preventing or removing any influence of environmental conditions on the non-sensor portions of the system. This, therefore, becomes an important design consideration.
Of course, the primary design consideration in developing a sensor array is the method by which information from each sensor can be separated for individual identification from among all of the information arriving at the central processing location on the single data stream. Distributed sensing systems developed previously have generally applied one of two approaches for separating information of an individual sensor from a single data stream.
One approach which has been used for separating each sensor's information from the single data stream has been to frequency-division multiplex the sensor outputs, in the manner described by I. P. Giles, D. Uttam, B. Culshaw, and D. E. N. Davies, "Coherent Optical-Fibre Sensors With Modulated Laser Sources," Electronics Letters, Vol. 19, Page 14, (1983). This approach is accomplished by frequency ramping the optical source and arranging the array geometry so that the transit time of the light from the source to a sensor and back to the central location is unique for each sensor. In this case, the array output is mixed with the sources's present output, thereby producing a unique central frequency for each sensor. The environmental information is carried in the sidebands about this central frequency.
One particular problem with the above-described system involves the "fly back" period when the periodic ramp signal is reset from its maximum to its minimum position. This fly back period comprises a time when system operation may not occur, since no ramp signal is present, and no meaningful results would be produced. This places some limit on the rate at which environmental conditions may change and still be reliably monitored by the sensor system.
Another problem with this particular system is that the number of sensors which may be used in the array or the frequency range of the signals to be detected are limited based on the range of FM frequencies which are utilized in the ramp signal, and on the period of the ramp signal. More specifically, since a different central frequency is produced for each sensor, the amount of difference between each such central frequency and the overall range of frequencies within which these central frequencies are contained dictates the number of sensors which may be utilized. Equivalently, the number of sensors, together with the overall range of frequencies determine the maximum difference between central frequencies, and hence the maximum environmental frequencies which may be detected. The range of frequencies is, of course, determined by the slope and period of the ramp signal.
These sensor configurations are also limited in the distance from the optical source which a given sensor may be positioned, not only due to the limitations based on the coherence length of the optical source, but also based on the fact that as the sensor is moved further from the optical source, the path length difference between adjacent optical paths becomes very large.
Another approach which has been used for separating each sensor's information from the single data stream comprises time-division multiplexing of the sensor outputs, as is described by M. L. Henning et al., "Optical Fibre Hydrophones with Down lead Insensitivity," I.E.E. Conference Publication 221, pages 23-27, (April 1983). In time-division multiplexing, the optical input most generally is pulsed so that the input signal comprises a pulse waveform. In the interferometric pulsed system described by Henning et al., the input light is pulsed twice with a particular delay between the two pulses. This delay is determined by the geometry of the sensor, and in particular by the relative delay between the two arms of the interfometer comprising the sensor. Specifically, the optical input pulses communicated through each sensor are mixed and placed on the output fiber by each of the sensors at a different time. By controlling the relative position of the sensors, interleaving of the pulse signals may be accomplished as the signals are multiplexed from the sensors onto a return fiber bus. These interleaved pulse signals are then carried back to the central processing location where demultiplexing and further signal processing occur.
One of the problems with these types of systems is that they generally have required use of an optical source having a coherence length which is longer than the path length difference between adjacent signal paths. The long coherence length is necessary in order to have the light from adjacent paths interfere. The interference creates an intensity modulation which is proportional to the phase modulation created in the light by the environment. In addition, the two pulses which are launched into the sensor array are generated from the source at different times. The result of mixing light which originates from the source at different times is phase induced intensity noise. Such source phase induced noise may create a limitation to the sensitivity of a sensor in such a system. Another limitation with these types of devices is that they measure only the difference between the sensors, and do not provide a means for measuring the environmental effects on a selected sensor by itself.
Based on the above, it would be an important improvement in the art to provide a sensing system and technique for multiplexing a plurality of remote sensors without being subject to the above-identified restrictions. Thus, the system should optionally be free of sensor spacing limitations, and experience little degradation of the signals carried thereon due to laser phase-induced intensity noise. Such a system should provide for operation without requiring use of electronics or active devices in the environmental sensing region. The system should provide for maximized duty cycle operation to increase the efficiency and potential applications of the system. Preferably, such a system should permit use of any of a wide range of optical sources, including short or moderate, as well as long, coherence length sources where the coherence length is greater than or equal to about one centimeter, and should be both simple and economical to produce and use in actual application.